Update on Week 3–4 Severe Weather Project

Update on Week 3–4 Severe Weather Project

9 February 2018

Week 3–4 Severe Weather Project

 Forecasts for weeks 1–2 are just a stepping stone.

 Work is under progress for weeks 3–4 using the CFSv2 45-day hindcasts and real-time forecasts.

Methodology

 Hybrid dynamical–statistical forecast model

 Using the Supercell Composite Parameter (SCP) from the GEFS forecasts as a predictor

 Week 1–2 SW forecast based on the statistical relationship between model SCP (GEFS hindcast) and OBS LSR (Local Storm Report), including hail, tornado, and damaging wind

 Forecast improvements:

• 5^o×5^o area-average anomalies

• Adjustment for the amplitude of forecasted anomalies

 Forecast skill (AC) assessed with cross-validations over the 1996–2012 GEFS hindcast period

SCP (Supercell Composite Parameter)

SCP = (CAPE/1000 J kg^1)×(SRH/50 m^2 s^2)×(BWD/20 m s^1)

 CAPE: convective available potential energy

 SRH: storm-relative helicity

 BWD: bulk wind difference

Week 1 GEFS Skill for SCP

AC: GEFS vs. CFSR 1996–2012

GEFS Hindcasts

 16-day forecast

 1996–2012 (17yrs)

 4 days apart

 5 members

 0.5^o×0.5^o resolution

Week-2 skill is much lower than the Week-1.

Cross-validation over MAM 1996–2012 5^o×5^o area average

Hybrid Model Forecast Skill for LSR3 Anomaly Correlation

Relationship between GEFS SCP and OBS LSR3

LSR3: Hail + Tornado + Wind

Tornado Outbreaks May 24–30, 2011

Week-1 Forecast LSR3 Week-2 Forecast LSR3

Week-1 Forecast (Adjusted) Week-2 Forecast (Adjusted) 5^o×5^o 0.5^o×0.5^o

OBS Weekly LSR3 OBS Weekly LSR3

Forecast = Anomaly_FCST

+ Climatology_OBS

Adjustment =

Anomaly_FCST × Ratio + Climatology_OBS Ratio = SD_OBS/SD_FCST

GEFS Real-time Forecast for SCP

Weekly SCP is the 7-day average of daily mean SCP from 12Z to 12Z.

Day 1 12Z

18Z

Day 0 00Z

06Z

Day 0 12Z

20 x 4 = 80 members

Week 1 Week 2

GEFS 80-member Mean Forecast

04Feb 12Z 18Z 05Feb 00Z 06Z

Week 1

Week 2

SCP 5

×5

Day 1

Day 2

Day 3

Week-1 Forecast LSR3

Week-1 Forecast Category and Probability

Week 1:

5–11 FEB

2018 80-member

ensemble

mean forecast

Based on the distribution of the 80-member forecasts

Probabilistic Outlook

 Distribution of LSR3 hindcasts (or observations) determines the thresholds for above-normal, near-normal, and below- normal categories, each 33%.

 The distribution of individual 80-member forecasts in the three categories provides the probabilistic forecast for LSR3.

In few months, we can start providing real-time week 1–2 severe weather forecasts based on GEFS to OPB.